Test structures are commonly used on silicon slices to enable the continuous monitoring of the slices as they are processed. The diffusion of impurities into a silicon slice may change its conductivity from N-type to P-type or vice versa and decrease its resistivity, and a measurement of resistivity can indicate the level of impurity diffusion. For example, in a typical solar cell process a region of N-type impurity having a sheet resistance of 60 ohms per square is diffused into a P-type silicon slice to a depth of 0.01 mil (thousandth of an inch). The resistance of the diffused region provides an indication of the density of impurity diffusion. The bulk resistivity of the undiffused silicon is typically one to 10 ohm-cm and typically has a resistivity several orders of magnitude greater than the diffused region. A typical test structure includes a series of metal terminals or contacts deposited at spaced locations along the slices. The resistance measured between a pair of adjacent contacts, enables the monitoring of the sheet resistance of the silicon, and therefore the processing step. Such test structures are useful on a variety of semiconductor devices, for example in the manufacture of solar cell material.
One type of test pattern of deposited contacts includes a series of parallel metal strips, each contacting a diffused region of a silicon slice which is defined in the surface of the slice by oxide masking. The metal contacts are not allowed to extend to the edge of the diffused region, so that the resistance measured between a pair of contacts is affected by the indefinite region beyond the ends of the bars along which currents may move in passing between the contacts. Also the surface of the P-region surrounding the N+ region can invert, so as to give erroneous readings of resistance in the N+ diffused region. A simple test pattern which enabled the measurement of the resistance of a semiconductor material for currents passing through a known limited area, would enable such measurements to be more accurately made in a wider variety of situations.